NFS: move nfs_server flag initialization

Make it possible for the NFSv4 mount set up logic to pass mount option
flags down the stack to nfs_create_rpc_client().

This is immediately useful if we want NFS mount options to modulate
settings of the underlying RPC transport, but it may be useful at some
later point if other parts of the NFSv4 mount initialization logic
want to know what the mount options are.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>